WASHINGTON (Reuters) - President Donald Trump's call for Attorney General Jeff Sessions to investigate former Democratic rival Hillary Clinton is "highly inappropriate" and threatens to erode the separation between law and politics, a leading Republican senator said on Tuesday.

"Prosecutorial decisions should be based on applying facts to the law without hint of political motivation. To do otherwise is to run away from the long-standing American tradition of separating the law from politics," Senator Lindsey Graham said in a series of posts on Twitter.

"President Trump's tweet today suggesting Attorney General Sessions pursue prosecution of a former political rival is highly inappropriate," said Graham, who defended Sessions from criticism by the president.